Implement Persistent Storage

[bschwind]

I think this is not technically part of the spec, but it's very much a desired feature. Given that this is intended to be a simple-to-operate server, I would likely start with either a custom single-file-based storage approach (with a serde-serialized struct containing all desired state), or an sqlite database.

If this broker ever grows into a multi-node broker cluster, we can start thinking about storage solutions which are more appropriate for distributed systems.